The present invention generally relates to electronically controlled aircraft starter-generator systems, and more particularly starter-generator systems that may be employed under widely varying environmental conditions.
Modern aircraft are designed with electronically controlled engine start systems which use power converters to regulate the power delivered to a start motor. When the engine has been exposed to cold ambient temperature, the viscous drag caused by cold engine gearbox oil increases the amplitude of starting torque which must be supplied to reasonably accelerate the engine through light-off. Conversely when the engine temperature is moderate or hot, the required amplitude of starting torque which must be supplied is lower.
The electrical power supplied to the start motor may be converted and controlled by one or more power converters to produce a desired torque profile (i.e., torque vs. rotational speed of the engine). The torque profile must be suitable to successfully start cold soaked engines, as well as engines in high temperature ambient conditions. This means that the power converters must be thermally designed to provide the maximum starting torque even when the engine ambient conditions are such that the engine could be started with much lower starting torque.
The power converters may include high power electronic devices which experience a temperature rise at a rate that is a function of starting power applied to an engine. The junction temperature of the power electronic devices must be carefully managed in order to achieve acceptable power converter reliability. Typically, temperature rise of the device junction may be mitigated by incorporating fans and/or large heat sinks in the start controller. In some cases, a start controller may be constructed with oversized power electronic devices which do not heat rapidly or which may tolerate high temperature.
It can be seen that there exists a need for such a starting system that can successfully operate in both hot and cold ambient conditions. Furthermore there is a need for an engine starting system that may employ a start controller that does not require fans, large heat sinks or oversized power electronic devices.